Biaxially stretched polypropylene films are excellent in transparency, mechanical properties, electrical properties, and the like and therefore have many applications such as packaging, tapes, and electrical uses including cable wrapping and capacitors.
As for capacitor applications, the films are particularly preferably used in high-voltage capacitors for direct current use and for alternating current use because of their excellent dielectric strength properties and low-loss properties.
Recently, various types of electric equipment have been incorporating inverters, and along with this trend, demands have been growing for capacitors smaller in size and enhanced in capacitance. Because of these demands from the market, in particular from automobile applications (including a hybrid car application), a biaxially stretched polypropylene film that is thinner and is also improved in dielectric strength and processability into an element is essential.
From the viewpoints of dielectric strength and processability into an element, it is necessary to make the surface of the biaxially stretched polypropylene film moderately rough, which is particularly important in order to improve the smoothness and oil impregnation of the film or to ensure the protective properties of a metallized capacitor. The protective properties herein mean a function, of a metallized capacitor the electrode of which is a metallized film formed on such a dielectric film, to allow the deposited metal to scatter at the time of overdischarge due to discharge energy so as to recover insulation and to prevent a short circuit, thereby maintaining the functions of the capacitor or preventing dielectric breakdown of the capacitor. The protective properties are also an extremely useful function from the safety viewpoint.
As a method of achieving a rough surface, mechanical methods such as embossing and sandblasting, chemical methods such as chemical etching with a solvent, a method of stretching a sheet containing polymers of different species such as polyethylene, a method of stretching a sheet in which β crystals have formed (see Japanese Patent Application Laid-open Nos. 51-63500 and 2001-324607, for example), and the like are conventionally suggested.
However, these may not necessarily be adequate in terms of roughness density, irregularly large protrusions, and the number of protrusions because roughness density is low in the mechanical methods and the chemical methods and, in the method of stretching a sheet in which β crystals have formed, irregularly large protrusions readily form. In addition, films with rough surfaces that are formed by these methods tend to cause insufficient oil impregnation between film layers at the time of capacitor fabrication to leave some areas not impregnated, which can impair the lifetime of the capacitor. In the method of stretching a sheet containing polymers of different species such as polyethylene, although capacitor fabrication leaves fewer air bubbles, the polymers of different species can adversely affect at the time of film recycling to impair recyclability.
Besides, the biaxially stretched polypropylene film obtained by any of these methods may not achieve adequate protective properties under stringent conditions with a potential gradient of 350 V/μm or higher where the capacitor is used, which can lower reliability. The potential gradient herein is the voltage applied to a dielectric film, after divided by the thickness of the film, that is, voltage applied per unit film thickness.
On the other hand, in terms of roughness density and uniform protrusions, a polypropylene film with high melt tension (see Japanese Patent Application Laid-open No. 2001-72778, for example), a film in which a polypropylene film with high melt tension and a usual polypropylene film are stacked (see Japanese Patent Application Laid-open No. 2001-129944, for example), and the like are suggested. When a polypropylene resin with high melt tension is used in a capacitor, however, heat resistance and pressure resistance cannot be adequate due to the resin structure and therefore dielectric breakdown voltage significantly decreases in particular at high temperatures. In addition, by the technique of stacking a polypropylene resin with high melt tension, especially in the case of a thin film having a film thickness of 5 μm or smaller, it is very difficult to achieve a stacked structure with uniform thickness, and the fact of the matter is that the dielectric film to be obtained is poor in uniformity and therefore cannot be satisfactory in practical use.
Japanese Patent No. 3508515 discloses a biaxially stretched polypropylene film in which the degree of surface roughness is controlled and a method for fabricating the same. However, this method is inadequate and is less prone to roughen both surfaces of the film finely and to control the protrusion height for both surfaces of the film.
Japanese Patent Application Laid-open Nos. 2007-308604 and 2008-133446 where roughness is specified for at least one surface of the film claim that a finely roughened surface formed by specifying the β crystal fraction of a cast raw sheet to be within a certain range can have windability into an element and pressure resistance in balance. This fabrication method, however, cannot adequately control the degree of surface roughness for both surfaces of the film, and the finely roughened surface of the resulting film is yet to satisfy requirements for pressure resistance and processability into an element particularly in automobile applications.